This invention relates to an exposure method and an exposure apparatus for use in the manufacture of semiconductor devices or magnetic heads, for example. Particularly, the invention is directed to improvements in a shape of a resist pattern after exposure or development.
Generally, when the numerical aperture of a projection optical system is denoted by NA and the exposure wavelength used is xcex, the resolution and the depth of focus can be expressed as follows:
Resolution=k1vxc3x97xcexNAxe2x80x83xe2x80x83(1)
Depth of Focus=k2k2xc3x97xcex/NA2xe2x80x83xe2x80x83(2)
In equations (1) and (2), k1 and k2 are parameters of the resist. In order to improve the resolution under a predetermined k1 and k2, from equation (1) it is seen that the NA should be enlarged. On the other hand, enlargement of the NA results in a decrease in the depth of focus, as seen from equation (2). Therefore, in accordance with a pattern size and shape desired, there will be an optimum NA to be selected.
This similarly applies to the selection of a value of "sgr" of an illumination system. If, for a resist of a particular film thickness, printing is made while changing the "sgr" of the illumination, there will be cases in which the shape of the resist is improved with a smaller "sgr" of the illumination system and more definite perpendicular pattern shaping is attained. Which "sgr" of the illumination system is good is similarly determined in accordance with the size and shape of the pattern desired.
Conventionally, in consideration of the above, exposure apparatuses are provided with plural values of NA and "sgr" of the illumination system, and a particular combination of NA and "sgr" is selected in accordance with the size and shape of the pattern desired.
It has recently been found that, to a resist of a particular type, a dual exposure procedure based on two combinations of NA and "sgr" is effective to produce a higher image contrast. If the resist thickness is large, even for ordinary resists, there is a tendency such as described below.
FIG. 1 illustrates resist profiles after development, as formed by printing under various conditions. FIG. 1 at (a) shows the result of exposure with a moderate NA a large illumination "sgr", wherein the top end of the resist pattern is narrowed. FIG. 1 at (b) shows the result of exposure with a moderate NA and a small illumination "sgr", wherein the top end of the resist pattern is rounded, contrary to that of FIG. 1 at (a). FIG. 1 at (c) shows the result of dual exposure made with different exposure amounts under the FIG. 1 at (a) and FIG. 1 at (b) conditions, respectively. The result is a best profile, intermediate of them.
It is an object of the present invention to provide an exposure apparatus and/or an exposure method, by which exposure of the same exposure region can be performed under different illumination conditions and by use of different projection optical systems.
It is another object of the present invention to provide an exposure apparatus and/or an exposure method, by which, with an improved exposure amount control and an optimized exposure sequence for dual exposure, an improved projection exposure process is enabled.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.